Expanding Tolerance Analysis for a Robust Product Design Expanding Tolerance Analysis for a Robust Product | Page 3

Expanding Tolerance Analysis for a Robust Product Design

minimum insuring that such mistakes don’t lead to safety
issues or equipment failures.

2.6. Kinematic Effects
Many assemblies contain parts aligned using feature of
size, such as alignment pin inserting into holes and slots.
Kinematic variation occurs due to motion between parts
resulting from clearances required for assembly. Of course
products with moving parts will need to be examined at
multiple positions throughout the range of motion to insure
they will perform as designed.

2.7. Manufacturing Processes
For any given part type and desired material there are
numerous manufacturing options. Increased accuracy and
precision on dimensional characteristics of the part result in
higher costs, so the challenge of the engineer is to select the
most accurate and precise method required for the design
through the definition of the least restrictive tolerances
possible. Ideally acceptable assembly-level variation should be
distributed to part-level dimensions so that cost is optimized
through the use of the overall least expensive set of
manufacturing processes possible. To do this effectively
requires evaluation of the assembly requirements; it cannot be
done solely using part by part analysis.

3.0. Six Steps to Robust Designs
There are six (6) primary steps to achieving a robust design.
They are:
1. Identify the requirements
2. Create conceptual design (often several)
3. Identify critical functional features
4. Understand sources of variation
a. Forces
b. Thermal
c. Manufacturing and Assembly methods
d. Etc.
5. Iterate changes in design
a. Nominal
b. Tolerances
6. Document assembly and part requirements

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3.1. Identify the Requirements
Requirements for a design come from many sources. For
many companies, requirements are initially provided by
market research such as a new product to fulfill an identified
market need. The choice of materials is often based on cost or
a specific look and feel of a product rather than just the
material properties required for strength or flexibility.
Customers provide direct feedback based on use of a
product in either the intended method or by using the product
in ways not thought of by the designers.
Prior products are one of the largest sources of design
requirements. Completely new products are rare compared to
the many iterative changes that occur with existing products.
Experience gained from manufacturing data provides direct
input to the new product design.
Manufacturing may influence the design requirements by
addressing the cost of different manufacturing methods or the
ability to hold certain design specifications and tolerances.
Finally, various regulatory requirements and existing
standards impact design specifications. Federal, regional,
local and specialized government agencies can directly impact
overall design.
Standards committees and industry
associations also provide guidance that can impact design
requirements.

3.2. Create Conceptual Design
Conceptual Design creation is the first stage in the
interpretation of the requirements into a representation of the
new product. The conceptual design can be drawn on paper or
white board or inside a CAD system as a 2D sketch or a solid
model. It is often helpful to consider several different
alternatives with different permissible ranges of important
components of the design. Evaluating the characteristics of
these optional approaches will allow the exploration of how
sensitive each is on the important functional requirements.

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